The present invention relates to a double negative cutting insert intended for tools for chip removing machining of the type that comprises a body with a flat, polygonal basic shape which has opposite upper and lower sides of identical shape. Those sides are turned from an imaginary neutral plane through the body and transverse flank surfaces extend between those sides adjacent to which material-removing major cutting edges are formed. The major cutting edges extend from the area of a corner and co-operate with a minor cutting edge formed adjacent to a parallel land. The minor cutting edge has the purpose of planar machining the surface of a work piece which is left after material removal.
Double negative and double-positive cutting inserts have advantages as well as disadvantages. A double negative insert is an indexible negative insert having cutting edges on its top and bottom sides. The double negative cutting inserts of conventional type are advantageous because cutting edges may be formed adjacent to the two opposite sides of the cutting insert body. In this way, the cutting insert may be formed with a generally doubled number of cutting edges in comparison with double-positive cutting inserts of the corresponding type. This means that a large number of cutting edges may be used before the individual cutting insert has to be discarded. This is something, which in turn reduces the purchase costs of the user. However, a disadvantage of the double negative cutting inserts is that they have mediocre cutting performance. This is something, which means impaired operating economy. Double-positive cutting inserts, on the other hand, have good cutting performance and enables quick and efficient working, but such cutting inserts may be formed with cutting edges only along the top side thereof. This is something which limits the number of feasible cutting edges.
Previously, cutting inserts have been developed which combine the conventional advantages of the double negative and double-positive cutting inserts, respectively, without the defects of the above-mentioned disadvantages. That is due to the cutting insert being endowed with a double negative basic shape at the same time as the top and bottom sides of the cutting inserts are formed with countersinks adjacent to the cutting edges in order to bring about a positive cutting geometry during operation. Previously known combination cutting inserts of this type have, however, had a square or hexagonal shape. As an example of such a combination cutting insert, a cutting insert should be mentioned which is commercially available under the trade mark TRIGON and is marketed by Greenleaf Corporation, Greenleaf Drive, Saegertown, Pa. 16433.
An aggravating disadvantage of square as well as hexagonal cutting inserts is, however, that the corner angle at each active working corner becomes moderate. Thus, in square cutting inserts, the corner angle is 90xc2x0 and in the hexagonal cutting inserts of the type TRIGON, the corner angle is even smaller. This means that the active corner becomes considerably acute, whereby the corner portion of the cutting insert risks to be broken up if it is submitted to considerable forces during operation.
U.S. Pat. Nos. 3,497,933, 4,954,021 and 4,966,500 show different embodiments of cutting inserts having a generally octagonal basic shape; i.e. cutting inserts which in one way or the other way have eight edges.
The present invention aims at obviating the above-mentioned inconveniences of previously known combination cutting inserts and at providing an improved cutting insert. Therefore, a primary object of the invention is to provide a cutting insert, which may be formed with a large number of cutting edges adjacent to active corners of a strong and robust character, and that in spite of this should be able to be fixed in a steady and reliable way in a corresponding insert seat. Although the cutting insert according to the invention should be able to be used for a wide variety of tools for chip removing machining, such as turning tools, the insert is primarily intended for milling tools, in particular face mills and side-milling cutters, regardless whether these for the mounting of the cutting inserts make use of cassettes or not. In a particular aspect, the invention aims at providing a cutting insert which is optimized for milling tools of the type that is either right-hand cutting or left-hand cutting, the purpose being to bring about the largest feasible number of useful strong cutting edges for either of these two working cases. In another aspect, the invention aims at providing a cutting insert which is xe2x80x9cneutral,xe2x80x9d to the extent that one and the same cutting insert should be able to be used for either right-hand cutting working or left-hand cutting working; all with the purpose of obviating the need of separate stock-keeping and distribution of two different types of cutting inserts. Another aim of the invention is to create a cutting insert, which may be manufactured in a simple way, above all in respect of the ability to release from a mold after molding. Another aim is to create a cutting insert which may be formed with an aperture intended for a fixing screw, which aperture has a diameter which is large in relation to the dimension of the cutting insert, the outermost aim being to enable manufacture of small cutting inserts with many cutting edges which may be fixed by means of simple, conventional fixing screws.
According to the invention, at least the primary aim is attained by a double negative cutting insert for chip removing machining. The insert includes a body of generally flat polygonal basic shape. The body has opposite top and bottom sides of identical shape, and eight transverse flank surfaces extending between the top and bottom sides, wherein the polygonal shape is octagonal with eight obtuse-angle corners. The corners comprise four active cutting corners and four inactive cutting corners. Two of the flank surfaces approach one another at each active cutting corner and are spaced from one another by a pair of lands. Each flank surface forms a major cutting edge at the top side and a major cutting edge at the bottom side. Each land forms a minor cutting edge at least at one of the top and bottom sides, wherein each active cutting corner at the top side includes at least one major cutting edge and at least one minor cutting edge. Each of the top and bottom sides includes four upstanding supporting projections disposed adjacent respective ones of the inactive corners. Each supporting projection includes a planar top surface. The top surfaces of the supporting projections on the top side lie in a common plane oriented parallel to a neutral plane of the body disposed midway between the top and bottom sides. The top surfaces of the supporting projections on the bottom side lie in a common plane oriented parallel to the neutral plane. Each of the top and bottom sides includes areas disposed at each active corner between respective pairs of adjacent supporting projections. Those areas comprise concave lands to provide a positive cutting geometry during cutting operations.